Runabout Fuel Tank Options

Questions about modifying a design

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Stromz
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Runabout Fuel Tank Options

Postby Stromz » Fri Oct 07, 2016 2:43 pm

Would like to hear from builders regarding fuel tank options for the Malahini or runabouts in general and where they're getting installed...not interested in portable plastic tanks. And then the next concern is where to locate. In my Glastron CVX-16 a 16 gallon tank is located forward under the deck which is where I'm thinking for the Malahini. Don't want to guess on this one and find out the weight/balance creates other issues. Would love to hear what others have done with their fuel tanks relative to: shape, material, size, location and installation method. And of course how did it impact performance and handling. I would also not have a problem utilizing a multi-tank arrangement which would allow for a more dynamic load and balance situation depending on the need. Obviously I don't want to over-design or over-complicate my approach. Feel free to take shots at my comments and thoughts. I'm in the early stages of getting my head wrapped around various build stages similar to when I built the plane and the fuel tank arrangement is yet another part of the puzzle that I'd like to get put in it's proper place. By the way, I like what I'm reading in this forum. There's no doubt a lot of knowledge floating around...I like that.
Last edited by Stromz on Fri Oct 21, 2016 12:20 pm, edited 1 time in total.

Kevin Morin
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Re: Runabout Fuel Tank Options

Postby Kevin Morin » Fri Oct 07, 2016 9:43 pm

Stormz, I'm not a wood boat poster and the builders of this design will be more informed than I- by a long way. So my remarks are just a reminder of things you probably already realize? I'll post them anyway and hope they're not just nuisance?

The plane's tanks being in the wings didn't effect the weight and balance like they will in a boat located in the ends of the hull - from full to empty. What I'm pointing out is the wings being a fixed point in the fore aft trim- regardless of state of fill- those tanks contributed pretty similarly to the trim of the nose/bow- up and down due to their fixed location or "moment arm".

Any planing hull has a couple of dynamics to consider- like a plane sitting on the landing gear versus flying. If a tank is located in the ends of the hull- the pitch of the bow takes on two moment conditions. First when the tank if full and second as the tank empties fuel. So if a bow ( located ) tank is filled it helps trim down the bow but that trim/moment is lost when you run the tank out/low.

Likewise if the tanks are aft the center of buoyancy toward the stern- then at rest this pitches the bow upward (depends on overall wt & balance) but when the stern located tank is low/dry the bow up pitch is lost by comparison.

So, planning a tank or multiple tank locations needs to consider the all up wt (tank and fuel) and location fore or aft the CB so the moment of that wt can be considered. If a planing boat is balanced WithOUt fuel (?) then locating the tank as close a possible to the CB along the centerline is considered the most 'neutral' position for that dynamic load. This is somewhat like putting fuel in the wings of a light plane- the moment of pitch is reduced regardless of fill state due to the location being close to the CG/CLift.

The complication in a boat is; the somewhat radical shift of the trim/balance once a hull planes. THEN, the hull is balanced on the leading edge of the running waterline; not the at-rest waterline. This moves aft as the speed moves up. So in this consideration of tankage's trim effect- a designer usually finds a few different running waterlines in the Plan View and considers balance fore and aft with this approximate station in mind.

Not answering your questions from a builder of this model, I'm simply making some general remarks about marine tanks as contribution to trim and considering both full and empty tank mass when planning.

My impression of this class of planing hull is: I'd want to consider the tank(s) as close to the CB as possible so that at rest the hull was not pitched up or down in relation to tank fill state- and then I'd move the tanks 5-8% aft of that point (not seeing framing plans this is purely speculation) so they were still close to a 'neutral' pitch moment when running? And by staying closer to the CB than a proportional waterline change when planing- the tanks would contribute to a bow down or balance pitch by the bow when running. This would allow the cav plate, trim tabs or other trim to be the final 'control' in regard bow pitch.

Just a few remarks, from a metal boat builder- so lots of "salt" could be taken with my remarks on a design whose lines I've not studied or done a wt. & balance.

Cheers,
Kevin Morin
Kevin Morin

BillW
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Re: Runabout Fuel Tank Options

Postby BillW » Sat Oct 08, 2016 9:29 am

This is interesting to note:

I have a production boat with a welded aluminum tank, and it (the tank) has a placard that says:
"Do not install in the foreword half of the boat length" Presumably, that is because of the high G's encountered in
the front, in rough water. More so, than in the back, at high speed.

I also have two homebuilt boats with Moeller brand, built-in, plastic tanks. They do not have that limitation.
But, I did not install them in the front anyway.

A side note: the Moeller tanks are translucent plastic. So, if it is installed where it can be seen, then you
can see how much gas is in there.

Stromz
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Re: Runabout Fuel Tank Options

Postby Stromz » Sat Oct 08, 2016 12:01 pm

Kevin Morin wrote:My impression of this class of planing hull is: I'd want to consider the tank(s) as close to the CB as possible so that at rest the hull was not pitched up or down in relation to tank fill state- and then I'd move the tanks 5-8% aft of that point (not seeing framing plans this is purely speculation) so they were still close to a 'neutral' pitch moment when running? And by staying closer to the CB than a proportional waterline change when planing- the tanks would contribute to a bow down or balance pitch by the bow when running. This would allow the cav plate, trim tabs or other trim to be the final 'control' in regard bow pitch.
Cheers,
Kevin Morin

Kevin - Thank you for your insight. The dynamics of what we're talking about here are very similar to airplanes...the difference being if you don't get it right in a plane it'll kill you quickly. The wing tanks located roughly at CG (fore & aft) didn't have much in the way of pitch but certainly with roll just like a boat and the outer wing tanks were especially big on roll feedback for obvious reasons. Your feedback about keeping the tanks at roughly the CG point or slightly aft makes sense. Appreciate your thoughts.
Last edited by Stromz on Fri Oct 21, 2016 12:21 pm, edited 1 time in total.

Stromz
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Re: Runabout Fuel Tank Options

Postby Stromz » Sat Oct 08, 2016 12:05 pm

BillW wrote:This is interesting to note:

I have a production boat with a welded aluminum tank, and it (the tank) has a placard that says:
"Do not install in the foreword half of the boat length" Presumably, that is because of the high G's encountered in
the front, in rough water. More so, than in the back, at high speed.

I also have two homebuilt boats with Moeller brand, built-in, plastic tanks. They do not have that limitation.
But, I did not install them in the front anyway.

A side note: the Moeller tanks are translucent plastic. So, if it is installed where it can be seen, then you
can see how much gas is in there.

Awesome! I've been picking up on those plastic tanks and I'm intrigued with them. And yes, a plastic see-thru would be good for reference on fill level. Will have to do a search on the Moeller tanks to see what options we have. Also curious as to how they attach. Thank you much for the feedback. D.

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Bill Edmundson
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Re: Runabout Fuel Tank Options

Postby Bill Edmundson » Sat Oct 08, 2016 1:21 pm

Forward placed tanks are tested to a different standard than aft placed tanks because if impact loads.

Bill
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Kevin Morin
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Re: Runabout Fuel Tank Options

Postby Kevin Morin » Sat Oct 08, 2016 3:41 pm

Stromz, glad to see that you're including a wide search in your planning stage about tanks' materials, test standards, location and contribution to the boat's final performance.

A note about tanks at or near the CG fore and aft- but not on the centerline/keel plane. Wing tanks or tanks located outboard near the chines (with a cross over to equalize or measured draw tubes to the filters so the pull to the same levels) will dampen roll in a boat. But these are harder to fit and finish so they're often more custom built- less rectilinear cubic production tanks. Most of the volume under the side decks/sheer clamp/guard deck is not used in skiffs. It makes a fine place to locate a tank, and as long as there is some care given to the fuel draw line resistances (equalizing) they will draw down very evenly. Also cross over lines can be used, but require anti-siphon hardware to insure the tanks don't fill one side and not the other?

Not sure of the volume you're planning but if a large centerline tank won't fit due to structural elements and the deck- then smaller wing tanks due have some aid in stability IF they can be plumbed to keep equal levels?

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

Stromz
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Re: Runabout Fuel Tank Options

Postby Stromz » Sat Oct 08, 2016 4:29 pm

Yep - My preference would be to keep it down the middle. Side tanks would require rebalancing at times and probably turn out to be somewhat impractical for the Malahini or similar design. 1 or possibly a 2nd tank as close to centerline as feasible and probably no further forward than the front seat (or there abouts) kind of makes sense. Going with a heavier than typical outboard would also lend to going perhaps further forward than normal. It'll be interesting to study the plans to see where we can go with available tanks or will I have to go custom aluminum? Not worried about it at this point...good food for thought! You guys have given me some things to think about and I very much appreciate the feedback. David.

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rbrandenstein
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Re: Runabout Fuel Tank Options

Postby rbrandenstein » Mon Oct 10, 2016 8:49 am

Stromz,
On my Malahini, I have 14.5 gallon Moeller mounted just behind frame 1 on port side. This is opposite the helm since I thought I would balance it out a bit side to side. The battery is on the starboard side, also behind frame 1.
The boat sits a bit low in the rear, especially since I bought the 4-stroke Yamaha, which is heavier than comparable 2 strokes.
Once underway or on plane, I do not notice any affect of the tank. I would believe that once on-plane the attitude of the hull is the same regardless of the weight. It just may take a bit longer to get there depending on weight and distribution.

I didn't want the fuel tank in the bow. I considered how to get a tank under the front seat, but nothing available would fit. You'd have to probably build a custom tank for that.
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Stromz
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Re: Runabout Fuel Tank Options

Postby Stromz » Tue Oct 11, 2016 4:13 pm

Bob - I don't have my plans yet. Can you give me a reference on where Frame #1 is? Also, was curious about the weight of your 4-stroker. My intention is to use the ETEC 90 which is a 2 stroke but heavier than past 2 strokes. The Evinrude comes in at 320 lbs.

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rbrandenstein
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Re: Runabout Fuel Tank Options

Postby rbrandenstein » Wed Oct 12, 2016 7:15 am

Here is a picture of frame 1. Frame 0 is the transom frame. I do not have the plans handy, but I think it is about 35" from the transom.
IMG_0654.JPG

plus the tank, a 14.5 gallon Moeller.
IMG_0664.JPG

An alternate arrangement, which I think would offer more capacity is a longer, lower tank centered just behind frame one. It might require a different arrangement of the motor well construction.

The Yamaha 70hp 4 stroke is 260#. When I decided to get a 4 stoke (the admiral wanted a quieter, less smoky experience) I selected this Yamaha since it offered the most horsepower in this range at the lowest weight. Many brands of newer engines in the 60-80hp range are over 300#. I trolled Craiglist for many months until I found a used one.

"JeffH" on this forum, built a Malahini with an ETEC 90 HP. Search for his build thread and you should find additional information on the engine.
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Stromz
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Re: Runabout Fuel Tank Options

Postby Stromz » Wed Oct 12, 2016 10:57 am

Outstanding photos! Thank you very much. I'm beginning to see there may be several options. Ideally, I agree with your statement about getting the tank under the seats. I will probably investigate that thoroughly. Also like extra capacity and the ability to shift the weight based on loading so I might come up with a dual tank approach...

And I agree with others that we should probably go no further forward than the CB point...hopefully the front seats are within that range. Thanks again for the photos and comments...extremely helpful. David.

bob smith
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Re: Runabout Fuel Tank Options

Postby bob smith » Fri Oct 14, 2016 2:39 pm

Not to minimize the aforementioned vagaries of CG calculations and the like but if you have a boat large enough to carry passengers, you will experience a "roving CG".
My observation is the largest passengers generally sit in the aft seats and usually gravitate to the same side as the driver. Go figure!!!
For the record I have a 36 gal. rectangular underfloor tank centered on the CG. No problems with it at all fuel levels.
Bob Smith
Chester, SC

Kevin Morin
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Re: Runabout Fuel Tank Options

Postby Kevin Morin » Sat Oct 15, 2016 11:46 am

all on this thread, I re-read the thread and wanted to remark about some of the observations. The hull's CB is the balance point when at rest in the water and the CG is the balance point out of water sitting on a transverse pivot (theoretical consideration). In the size skiff we're discussing the wt of crew moving about would shift the CG due to the persons' relative wt compared to the all up displacement. However the CB is not shifted unless the boat changes waterline length (or waterplane area perhaps better said?) and that mostly happens when the boat planes.

So, a tank location and volume contributes to the CG Moment Arm/fore aft location but not the CB location- as the influence on CG is the mass multiplied by the lever arm of the distance forward or aft of the CB (at the time). The CG shifts act around the CB which only shifts (aft in most planing hulls) as the waterline runs aft under the hull as speed and planing raise the boat upward. This of course shifts radically toward the bow stem when the same hull buries the bow in a swell face.

I was cautioning Stromz just about the design concept of locating tanks too far forward (one of his earlier remarks) to get a bow down trim using the tank's moment arm to lower the naturally raising bow. (pitch up by the bow) If the CB runs aft when on plane then the forward tanks' full state moment arm is kind of multiplied because its farther forward but neither the at rest or running CG is changed in the full tank condition. That shift aft only happens when the fuel is burned off lowering the mass and therefore the moment arm.

I agree that you could send a crew forward (discussing open seating skiff, not necessarily this design 's seating) and trim by the bow. And of course if a heavy displacement crew member, like me for example, camps out by the stern on the helm side, then she's more than likely down by the stern and heeled to that side too!

My remarks were an attempt to observe than in small light wt skiffs with good power to wt ratios, putting fixed tankage as close the floating center - or just aft- would minimize the change in CG that happens from full to dry tanks while reducing, at the same time, trim by the bow due to the CG shifts. That leaves the trim tabs or engine angle trim as the primary (bow pitch) trim control, not the tanks' fill state where moving crew could be the only means of adjusting bow trim.

Hope I didn't add confusion to the tank location vs trim discussion, I was trying to observe the two centers, floatation/at rest lift (CB) and gravity (CG) as two different considerations. Related & dynamic centers but with different considerations to 'balance' the two- (pun intended.)

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

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Bill Edmundson
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Re: Runabout Fuel Tank Options

Postby Bill Edmundson » Sat Oct 15, 2016 12:17 pm

Kevin

I agree with you. There are many aspects to tank placement. What do you balance to? full tank, half tank, or empty tank? This is where trim comes in. It's a moving target. So, you have a way to move with it.

Bill
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